chore: fix typo in the cross validation notebook (#1069)

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Fixes #<issue_number_goes_here> 🦕

Author: shobsi

notebooks/ml/bq_dataframes_ml_cross_validation.ipynb

@@ -272,7 +272,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## 2.1 Define KFold class and Train/Test for Each Fold (Mauanl Approach)"
+    "## 2.1 Define KFold class and Train/Test for Each Fold (Manual Approach)"
    ]
   },
   {